In situ immunohistochemical detection of free κ and λ chains in the CLL microenvironment. (A-B) In lymph node infiltrates of a κ-chain CLL case, a prominent population of plasmacytoid lymphocytes expressing the κ FLC is detected along with a lower number of lymphocytes showing expression of the clone-unrelated λ chain, both populating the same microenvironment. Scattered plasma cells with either κ or λ FLC expression are also detected (panels A and B insets). (C-D) A similar picture is observed in bone marrow lymphoid infiltrates of a λ-chain CLL case. Immunohistochemistry performed by the streptavidin–avidin-biotin complex using the 3-3′-diaminobenzidine chromogen (brown signal). Original magnifications ×200 for all panels and ×400 for insets.